Have you looked at the 1939 register on findmypast? In 1939 there were about 15 people in Cornwall with the name Drummond.
Just a tip but it's Stirling not Sterling. Sterling refers pound sterling.
Best of luck!

Hey there, could you look through the Gerald Grahams (there are less than 10) and see if any of them were travelling salesmen (or a similar job) as that was the description of my grandad's real dad. Much appreciated if possible.

Are your maternal and paternal grandparents from ethnic groups? There seems to be imbalance in the ethnic backgrounds of people who take DNA tests.
You can transfer your results to Gedmatch, as there is a feature on it to see if his parents are related.

You could try a new DNA test from the UK called LivingDNA. It matches up your DNA to specific parts of the UK. It's also fairly cheap so with a budget like that you could buy another test to find relatives (living DNA doesn't find relatives, I think).